The invention relates to AC/DC or DC/AC converters of the hexagon type. A 12-pulse conversion circuit of the hexagon type has been described in U.S. Pat. No. 4,255,784. The main attribute of the Hexagon circuit is that, with a single transformer it can meet the stringent requirements regarding the harmonic contents of the AC input currents drawn by the converter from the AC supply lines. In particular, as opposed to more conventional static converter circuits, there is no need for more filtering circuits.
The performance of the Hexago circuit regarding the harmonics rests upon the effective inductances which determine the commutation overlap angle of its semiconductor switches. The required commutation inductances can be incorporated into the Hexagon transformer in the form of leakage inductances, or by providing external inductances in the form of "outboard" inductors. The rating of the converter, as well as the size, weight and efficiency come into consideration. U.S. Pat. No. 4,255,784 shows how to reduce the AC line current harmonics by "stretching" the commutation overlaps with the help of relatively small commutation inductances. Outboard inductor schemes for the same purpose have been described, one in U.S. Pat. No. 4,366,532, another in U.S. Pat. No. 4,683,527.
However, with the afore-stated designs which have been applied to a 12-pulse converter, the dominant AC line current harmonics have been reduced from 9% (as typical for a conventional 12-pulse converter) to less than 3%, but not any further.
Further reduction of the AC line harmonics is possible by using a converter having an increased pulse number. See 1) B..R Pelly, Thyristor Phase-Controlled Converters and Cycloconverters, John Wiley & Sons, Inc. N.Y., 1971; and 2) J. Schaeffer, Rectifier Circuits, Theory and Design, John Wiley & Sons, Inc. N.Y., 1965. A natural approach consists in using with a 12-pulse converter the same technique as going from a 6-pulse to a 12-pulse converter. Then, the best form of a 12-pulse converter will be used to build a 24-pulse converter. This leads to taking the Hexagon as the first choice in minimizing the harmonics. Two converters will, then, be combined in series or in parallel, so as to obtain a 24-pulse system, thereby cancelling the dominant 11th and 13th harmonics. However, the much lower 23rd and 25th harmonics become the dominant harmonics in such a scheme. There is also a serious drawback, since this solution calls for two three-phase transformers.
The purpose of the present invention is to design a single-transformer 24-pulse converter having much reduced AC line current harmonics.
The afore-stated U.S. Patents are hereby incorporated-by-reference.